This invention relates to robotic manipulators, and more particularly, to such manipulators having drive actuators attached to a base and providing movement of an end effector with three degrees of freedom (DOF).
Typical industrial robots having several degrees of freedom have actuators distributed at the joints. This eliminates much of the mechanical transmission resulting in simpler, more accurate design. Electric motors, the most commonly used actuators, are by their nature very heavy, and thus comprise a sizable percentage of the overall structural weight of the robot arm.
Distributing the motors at the joints places them in high inertial positions, and their masses become inertial loads on some of the other motors. The majority of mechanical work done by some robots is accelerating and decelerating the motors on the manipulator arms.
Modern teleoperator designs centralize the actuators in a low inertial position. This reduces the inertia and coupling forces on the joints, resulting in a quicker manipulator with improved control performance. The torque is transmitted via cables, metal tapes, back drivable gear trains and/or torque tubes. These transmission methods are inaccurate due to low mechanical stiffness and backlash, but have relatively little friction. Accuracy, while crucial in autonomous robots, is relatively unimportant in teleoperators due to the existence of a person in the control loop.
The recent emergence of direct-drive technology in robot design underscores ever increasing accuracy requirements of robots. Unfortunately direct drive servomotors with high output torques are heavier still than an equivalent gear motor. As a result, a serial link direct-drive manipulator, such as the one disclosed in Asada, et al, "Control of a Direct-Drive Arm" ASME J. Dyn. Sys., Meas. and Control, Vol. 105, September 1983, has a very massive structure.
Work has been done recently on in-parallel actuated kinematic structures, including arms with up to three to six DOF. These structures have the potential of greatly improving the mechanical performance of modern robots. Before this can be accomplished, a more detailed kinematic analysis of specific linkages must be carried out, with the goal of developing a practical engineering design. It is unfortunate that linkages which have such potential advantages also have extremely complex geometries for which few kinematic algorithms have yet been proposed. The geometric relationships between joint space and cartesian end effector space, the Jacobian relationships and the location and nature of singularities must be studied before a new design can realize its potential advantages.
By transmitting torque via a linkage which can be very stiff, the accuracy of direct drive is preserved. Additionally, links can be selected so as to provide a "gear reduction", that is, reduce speeds and amplify torques. A planar five-link linkage having simple kinematics has been analyzed thoroughly. The principle disadvantage of the design is that it only provides 2 DOF. This linkage is described by Asada, et al., in "Analysis and Design of a Direct-Drive Arm with a Five-Bar-Link Parallel Drive Mechanism", ASME J. Dyn. Sys., Meas. & Control, Vol. 106, No. 3, (September 1984) and in "A Linkage Designed for Direct-Drive Robot Arms," ASME J. Mech. Trans., Vol. 107, December 1985. The addition of a third DOF is achieved by placing the entire mechanism on a rotating base, which is itself directly driven. With this design, the dynamics of the five-bar linkage can be completely decoupled resulting in improved control performance. However, the base rotation will always be dynamically coupled, and the advantages of fixed motors are reduced, as the 2 DOF mechanism becomes an inertial load on the base actuator. Another approach is to have the five-bar linkage move in a horizontal plane and place a small motor at the manipulator tip for vertical motion. This has the advantage of simplicity, and is considerably more compact than the large rotating base, but again reduces the advantage of fixed motors.
The present invention overcomes many of the described disadvantages of the known devices and designs. In particular, the present invention provides a 3 DOF closed-chain kinematic structure particularly well suited to robot manipulators. This structure has geometric and Jacobian relationships which are much more simple than other parallel actuated spatial structures with three or more DOF. Further, the present invention provides such a device having a large work space volume with singularities which can be restricted to the boundary of the work space.
The present invention generally provides the use of a spatial extension of the planar five-bar linkage by applying a differential-type input to one of the input links of the structure. This differential-type input uses two actuators to actively control a 2 DOF input link. A second single-DOF actuator is connected to another input link, with each of the input links being respectively connected to a separate output link. The output links are then connected together, with an end effector or gripper being associated with one of the output links. In the preferred embodiment, the three actuators are fixedly attached to a base and the input link connected to the 2 DOF actuator is connected to the corresponding output link by the equivalent of a ball joint, the other end of which output link is connected to the other output link by a universal joint. Finally, the remaining joint between an input and an output link is a single DOF pin joint. In alternative embodiments, these joints and actuators may be varied in configuration.
It will be seen that such a manipulator made according to the present invention provides the features and advantages described. Further, the present invention provides a structure which can achieve speeds and accuracies unattainable with similar serial link designs. Other features and advantages of the present invention will be realized from a consideration of the drawings and the following detailed description of the preferred embodiments.